In the fields of electricity and electronics, to attain compaction and high performance of devices, a circuit board having characteristics well balanced at a high level, such as heat resistance, thermal dimensional stability, mechanical properties, low hygroscopicity, fire retardance, high frequency characteristics and the like, is demanded. It is further demanded for the circuit board to attain processing of signals at a high speed and to have small thickness. Therefore, the insulation substrate used for the circuit board also has to satisfy the above-mentioned demands.
Conventional insulation substrates used in this field include substrates comprising a glass cloth in which an epoxy resin is impregnated (hereinafter also referred to as "epoxy resin-impregnated substrate"), polyimide films, fluorine-contained films and the like. However, the epoxy resin-impregnated substrates have high hygroscopicity and poor high frequency characteristics, and it is difficult to make the thickness thereof small. Although polyimide films have good heat resistances, they have high hygroscopicities and poor high frequency characteristics. Fluorine-contained films have poor adhesivenesses, so that it is not easy to mount electro-conductive pastes or to be plated when forming through holes.
To employ a non-oriented sheet of poly-p-phenylene sulfide (poly-p-phenylene sulfide is hereinafter also referred to as "PPS", and non-oriented sheet of poly-p-phenylene sulfide sheet is hereinafter also referred to as "PPS sheet") as the insulation substrate of a circuit board is disclosed in Japanese Laid-open Patent Application (Kokai) No. 56-34426, and to employ a biaxially oriented film of PPS (hereinafter also referred to as "PPS film") as the insulation substrate of a circuit board is disclosed in Japanese Laid-open Patent Application (Kokai) No. 55-36945. These techniques are highly drawing attention recently.
However, although PPS sheets when used alone have satisfactory thermal dimensional stabilities, low hygroscopicities, fire retardances, high frequency characteristics and the like, the temperature to which they can stand is lower than that of biaxially oriented films (i.e., they are likely to thermally deform at temperatures higher than glass transition points). Further, the larger the number of heating steps, the more the crystallization, so that the more the brittleness. In cases where a PPS sheet is used for a printed circuit board, the heat resistance and brittleness are made to be satisfactory to some degree by controlling the crystal size and the like, there is a problem that PPS sheet likely to thermally deform when heated quickly.
PPS films when used alone exhibit dimensional change due to heat shrinkage, so that circuits are likely to be shifted when heat is applied during the manufacturing process of the circuit board. Further, they are easily broken when forming through holes in laminated circuit boards. Thus, their application is limited in the fields wherein thermal dimensional stability is highly demanded.
On the other hand, the following laminates comprising PPS films are known.
(1) A laminate comprising a fabric sheet made of an aromatic polyamide, with which a PPS film is adhered by an adhesive is disclosed in Japanese Laid-open Patent Application (Kokai) No. 60-63158.
(2) A laminate comprising a fabric sheet which is not melted at 300.degree. C. and has a thermal expansion coefficient at 150.degree. C. of not more than 50.times.10.sup.-6 1/.degree. C. and a PPS film is disclosed in Japanese Laid-open Patent Application (Kokai) No. 1-95585.
(3) PPS molded sheets reinforced with a glass fabric sheet are disclosed in Japanese Patent Publication (Kokoku) No. 60-50146, Japanese Laid-open Patent Application (Kokai) No. 57-96024, and so on. Further, to use these sheets for printed circuit boards is disclosed in Japanese Patent Publication (Kokoku) No. 60-52943, Japanese Laid-open Patent Application (Kokai) Nos. 59-3991 and 58-80890, and so on.
(4) A laminate comprising a glass fabric sheet to which a PPS sheet is adhered by heat melt, and a circuit board based on this laminate are disclosed in Japanese Laid-open Patent Application (Kokai) No. 4-224941.
(5) A laminate Comprising a glass fabric sheet, a PPS sheet and a PPS film, in the order mentioned, the PPS sheet and the PPS film being adhered by heat melt, as well as a circuit board based on this laminate, is proposed in Japanese Laid-open Patent Application (Kokai) No. 4-232739.
However, the above-described laminates and glass fabric sheets have the following problems.
In the laminate described in (1), the layers are adhered via an adhesive, so that the heat resistance of the adhesive adversely affects the substrate (i.e., the heat resistance of the whole substrate is governed by the heat resistance of the adhesive). Therefore, the excellent characteristics of PPS cannot be well exploited. Further, since the resin is not impregnated in the fabric sheet, there are problems that the electro-conductive paste may be impregnated, metal plating cannot be carried out (it is difficult to plate a metal because burrs of the fabric are formed), and so on.
In the laminate described in (2), a PPS film and a fabric sheet are laminated by adhesion by heat melt. Therefore, although the laminate has an excellent heat resistance, the adhesion is poor, so that the film layer and the fabric sheet layer are easily peeled when a stress such as bending stress is exerted.
In the sheet described in (3) comprising a glass fabric sheet in which PPS resin is impregnated, cracks are likely to be formed when a stress such as bending or punching is exerted, so that the processing of the circuit board is problematic. Thus, its use in the field in which the small thickness of the circuit board is demanded is restricted.
The laminate described in (4) has also problems in mechanical properties as the sheet described in (3). Further, since the degree of impregnation of the PPS resin into the glass fabric is small, formation of through holes is problematic.
In the laminate described in (5) too, the degree of impregnation of the PPS resin in the glass fabric is low, so that the formation of through holes is problematic. Further, since the strength of adhesion between the PPS sheet layer and the PPS film layer is not high, if a strong impact such as punching is applied, they may be peeled, so that the applications of the laminate are restricted.